Regulation of circannual rhythms in reproductive efficiency by secretions from the pineal gland (probably melatonin) remains an enigma even through this subject has been the focus of an intensive research effort in recent years. The objectives of the experiments outlined in this proposal are to determine: (1) if exogenous melatonin can replace darkness as the cue to stimulate seasonal changes in reproductive competency, (2) if the effect of melatonin is exerted on the hypothalamo-hypophyseal axis, (3) to examine the interaction of estradiol-17 beta and melatonin in regulating release of LH and FSH, and (4) to determine the effect of season on numbers and affinity of receptors for melatonin and estradiol-17 beta. We have chosen to use sheep, a short-day breeder, as the animal model for the proposed studies. The initial experiments are designed to determine if daily administration of melatonin at 1600 hours will prolong the breeding season in intact ewes and alter the seasonal pattern of secretion of LH in ovariectomized, estradiol-treated ewes. Preliminary data suggest that melatonin-treatment will extend the length of the breeding season from approximately 5 months to something greater than 7 months in intact ewes. The second series of experiments are designed to determine the site in the body where melatonin acts. We will look for direct effects of melatonin on the pituitary and hypothalamus in vitro. Dissociated anterior pituitary cells will be cultured for 18 hours in the presence of melatonin, estradiol-17 beta, or melatonin plus estradiol-17 beta after which the ability of the cells to release LH and FSH in response to GnRH will be determined. Similarly, the ability of melatonin to alter the release of GnRH from the hypothalamus in response to norepinehrine and/or prostaglandin E2, putative physiological secretogogues for GnRH, will be determined. The final experiments are designed to examine the effect of season on affinity and numbers of receptors for melatonin and estradiol-17 beta. We anticipate that receptors for melatonin will remain unchanged but that we will observe a seasonal change in receptors for estradiol-17 beta. The proposed research is designed to enhance our understanding of the mechanisms regulating annual rhythms in reproductive competency. A better understanding of this area lead to new schemes for regulating fertility in humans, food-producing animals and pets.